Protective coatings can be either thermoplastic or thermosetting. The advantages of a thermoplastic coating such as a cellulose ester wood coating include ease of application, low toxicity, good substrate wetting and gap-filling capabilities, good solvent compatibility, easily sanded and buffed, and numerous others. However, such thermoplastic coatings are easily damages by solvents and in general do not have good scratch resistance.
Thermoset coatings, however, have excellent solvent resistance and hardness properties, but they also have serious disadvantages. Most are thermally cured and their use is limited to substress that are stable at the curing temperatures which can be as much as 230.degree. C. Moisture-cured systems have been used to overcome the use of high cure temperatures but these have prolonged cure times and have humidity requirements. The use of ultraviolet radiation to transform a photocrosslinkable thermoplastic coating into a thermoset coating represents a desirable technological improvement. Curing can occur over a period of seconds to yield a hard, durable coating. In this manner, the advantages of both thermoplastic and thermosettable polymers can be maintained.
It would be desirable to develop a photocrosslinkable cellulose ester coating which would preserve the advantages traditionally associated with the use of cellulose ester lacquers and after exposure to suitable radiation would possess greatly improved solvent resistance and hardness properties. Such a product would have an enhanced utility in wood finishing operations where it is desirable to apply a lacquer coating on the substrate that may be sanded, buffed, repaired, or even removed by solvent as desired during the finishing operation. As the final step in the process, the coated substrate is exposed to ultraviolet radiation whereupon the solvent-removable thermoplastic coating becomes an insoluble solvent-resistant thermoset coating.
Attempts have been made in the prior art to achieve such a desired coating, however, such attempts have major deficiencies. For example, the crosslinkable cellulose esters described in U.S. Pat. Nos. 4,112,182; 4,490,516; 3,749,592; and 4,147,603 do not provide a desired level of solvent resistance and hardness.
Photopolymerizable cellulose esters described in U.S. Pat. No. 4,565,857 display a wide range of reactivities. For example, cellulose acetate propionate grafted with 0.9 moles of 2-isocyanatoethyl methacrylate per mole of anhydroglucose units has a short pot life in the presence of peroxides or a photoinitiator such as Irgacure.RTM. 651 (Ciba-Geigy) and can gel unexpectedly.
Other cellulose esters such as cellulose acetate propionate grafted with 0.9 moles of m-isopropenyl-.alpha.,.alpha.'-dimethylbenzyl isocyanate per mole of anhydroglucose units fails to polymerize to any significant degree in the presence of peroxides or a photoinitiator. It requires the presence of vinyl monomers and is not "self-crosslinkable."
Coatings of this material alone fail to polymerize and are easily removed by solvents. They also fail to develop a sufficient degree of hardness because they do not react to the extent that a crosslinked polymer network is formed.
The present invention provides cellulose esters that are capable of self-crosslinking (i.e., without the need of a vinyl comonomer). The present invention also provides coatings based on such cellulose esters for wood, metal, plastics, and the like that have properties such as improved solvent resistance and pencil hardness.